The present invention concerns a plate and frame filter press. More particularly, the invention is concerned with a modification of a terminal frame utilized in such a press whereby the flow of filtrate through said press is altered in order to enhance the rate of filtration of difficultly filterable slurries.
Plate and frame filter presses are widely used throughout the chemical industry. Typically a plate and frame filter press consists of alternate solid plates supporting a filter means such as a filtering fabric, for example, in the form of a composite cloth-paper media or the like and hollow frames in which there is a provision for the accumulation of filter aid to prevent blinding of the filter means thereby facilitating the filtration of the slurry. Such presses have been widely used throughout the chemical industry for many years for recovery of solids only or the clarification of a liquid only or the recovery of both solids and filtrate. No matter how complex the results of filtration may be, the principles of these plate and frame filter presses with their alternating flush plates and frames and the filtering means are basically alike. After the filter means are in place between the plates and frames and a closing device has pressed them tightly together, then the material to be filtered is pumped into the press at the slurry inlet. It passes through the feed channel to all of the chambers formed by adjacent plates and frames where the filter means stop suspended solids and allows only the clear liquid to go through to the discharge channel, leaving the solids in the chambers.
Such a plate and frame filter press is described in U.S. Pat. No. 1,722,497. In this patent, it is seen that the plates and frames are provided with side lugs so that they can be positioned on the side bars of a press frame. Each of the filter plates and/or filter means per se is constructed and arranged to be engaged and clamped between two of the filter frames in the usual manner. Usually these filter plates are provided with a ribbed or waffled surface to support the filtering fabric and provide channels for the filtered fluid to pass along the face of the plate to the discharge ports of the filter plate. In this frame and plate assembly, the slurry is fed into the press by means of a top feed to each of the filter assemblies and the filtrate is collected therefrom by means of a pipe secured to the lower part or bottom of the filter press.
As noted above, the plate and frame filter press has remained basically the same through its evolution over the last one hundred years although the operating pressures have materially increased up to 250 psi and greater although normal operating pressures of 50-150 psi are used. The earlier metal plate and frames have been modified to take advantage of the newly developed engineering plastics which have further allowed for a wide diversity of structures with improved efficiency of filtration and reduced consumption of filter aid. In particular, the configuration of the plates and frames have been varied widely so as to provide not only for multiple feed ports but a washing capability whereby solubles can be dissolved or washed out of the filter cake while it still remains in the filter press chambers. Thus, there will be, in many instances, the presence of a wash channel separate from a feed channel which channels are provided through different corners of the plates and frames.
In the filtering of slurries, it is necessary that the filter means receive a layer of filter aid to ensure that each frame receive a minimal coverage in order to prevent the filter means from blinding. It has been found that after termination of a typical run on a plate and frame filter press, the front frames are completely filled with filter cake (the mixture of filter aid and filtered solids) with the thickness of the cake decreasing toward the rear of the press until the cake is approximately one-half the thickness of the cake in the front frames. For this reason, large quantities of filter aid are admixed with the slurry prior to filtration in order to ensure that the last frames in the rear portion of the filter press receive sufficient filter aid to prevent blinding of the associated filter means.
It is an object of this invention to provide a method to improve the filtration rate and/or filter aid consumption in the operation of a plate and frame filter press for the filtration of difficultly filterable slurries.